Conventionally, semiconductor devices including a silicon (Si) substrate have been mainly used. In recent years, however, particularly in the field of power semiconductor devices, attention has been focused on semiconductor materials of a hexagonal system such as silicon carbide (SiC) and gallium nitride (GaN), and development of semiconductor devices including these semiconductor materials has been promoted.
A power semiconductor device is a semiconductor element designed for use under application of a high voltage and a large current, and is required to have low losses. In this regard, silicon carbide (SiC) itself has a breakdown voltage which is higher by one digit than that of silicon (Si). Therefore, with the use of SiC, it is possible to maintain a reverse breakdown voltage even when a depletion layer in a pn junction or a Schottky junction has a small thickness. Thus, the use of SiC enables a reduction of the thickness of a device and an increase in a doping concentration. Therefore, SiC is expected to be used as a material for power semiconductor devices which have low on-resistance and low losses and withstand a high voltage.
In addition, intelligent power devices, inclusive of the conventional Si power semiconductor devices, which have a protective function against overcurrent and overvoltage have recently been brought into active use.
Patent Document 1 describes that a configuration in which a transistor to be protected and a pn junction diode whose withstand voltage is sufficiently low are provided on a single substrate enables accurate control of the breakdown voltage and reduction of the total area of the chip. Patent Document 2 relating to a metal-oxide-semiconductor field effect transistor (MOSFET) having a trench structure also describes a structure to increase the breakdown voltage of an oxide film on the trench bottom and to ensure the reliability for a long time. According to Patent Document 2, after forming a mesa region near the trench MOSFET, a pn junction diode made of n-type polysilicon and p-type polysilicon is formed, thereby making the withstand voltage of the pn junction diode lower than that of the trench MOSFET.
Further, Patent Document 3 describes that, in order to provide protection against overvoltage, a switching element and a pn junction-type avalanche diode of which the curvature of pn junction and the withstand voltage are lower than those of the switching element are formed on a single substrate. According to Patent Document 3, the gate potential of the switching element can be controlled based on detection results of an avalanche current of the protective diode.